Here are some 0.020" gaskets, however, one place it says they are steel and another place it says they are copper. Talk to a Summit tech and have him eyeball the pieces before placing the order......
http://store.summitracing.com/partde...2&autoview=sku

I'm unfamiliar with the 390 chambers and whether or not you have a flat place on the head to make a squish with, but I'd be more concerned with widening the squish clearance with a thicker gasket than I would be about lowering the compression ratio. Maybe one of the other Ford guys on here can elaborate.